Arcus Odyssey
Passwords for this MegaDrive game were erroneously listed in the Master Section of Sega XS magazine, and it later appeared on several older online lists of SMS games.

Bugs Bunny (Sega)
Listed as "coming soon" in "Master System Les Jeux 1992-1993". Also mentioned in issues 12 & 15 of Sega Force.
A GameGear game, Bugs Bunny in Double Trouble, was released but not until 1996, so this may be a different title.

Captain Planet (unknown)
Mentioned in a 1994 French mag (mag??, issue ??, month ??) as being one of the last Master System games in development.

CJ Elephant Fugitive (Codemasters)
Mentioned on last page of Micro Machines instruction manual alongside several screenshots, and also previewed in Sega Power issue 48 (SMS Power scan). Game Gear version was released.

Cliffhanger (Sony Imagesoft)
Reported as in development in issue 5 of Sega Master Force. GameGear version was released.

Cube Zone (Sega)
'Cube Zone' is mentioned in a late 80's Sega retail sales folder, with a suspiciously Galactic Protector-like description: "Wild NEW action from the outer reaches of space! Defend your moon against the evil alien fleet. Colorful, explosive graphics!". According to a 1988 Mastertronic catalogue, it was assigned the part number 5107.

Die Hard II (Grandslam)
Advertised on the inside-cover of The Flintstones manual, listed in "Master System: Les Jeux 1992-1993" and reviewed in issue 1 of Sega Pro (11/91) alonside a couple of screenshots (scan). Also listed for sale in adverts in several UK mags between 1991 & 1993.

Dinobasher featuring BigNose the Caveman (Codemasters)
Reviewed in Sega Power (March 1994) (scan). Prototype owned, dumped & released by Bock. A second copy including a box insert surfaced in Australia in early/mid 2007.

Dragon's Revenge (Tengen)
An interview with Dan Van Elderen, Tengen's Chief Operating Officer , in issue 5 of Sega Master Force (11/93), states that SMS, MD & GG versions of this were planned. Only the MegaDrive version was released.

Drop Zone (Codemasters)
Mentioned as "out soon" in issue 54 of Sega Power (Aug '94) & previewed in issue 5 of the UK Official Sega Magazine (scan1) (scan2). Also reviewed in issue 8 (scan). Unconfirmed rumours of proto copies have circulated for a while. GameGear version was released.

Duck Tales (Sega)
Mentioned in issue 1 of Sega Pro (Nov '91) and listed as "Coming Soon" in "Les Jeux Master System 1992". Possibly a working name for wither Lucky Dime Caper or Deep Duck Trouble.

Excellent Dizzy Collection (Codemasters)
Compilation of 3 Dizzy games (Dizzy the Adventurer, Go! Dizzy Go! and Panic Dizzy). Bock has an early & incomplete proto. Complete version released on Game Gear.

F-15 Strike Eagle
Mentioned in a Software First advert in issue 54 of Sega Power (Aug '94). GameGear (& MegaDrive) versions were released.

Fire & Forget
Confirmed by programmer Gil Espeche as being ported to the Master System as a test.

Global Golf (Codemasters)
Listed as "In the pipeline" in Sega Power (July '94) and briedly mentioned alongside a screenshot in Spanish mag Hobby Consoles (SMS Power scan). Possibly became 'Ernie Els Golf' on the Game Gear?

Godfather, The (US Gold)
Reviewed by German mag Video Games (off-site scan) in February 1993 with several screenshots and also advertised as "coming soon" in a US Gold ad in issue 12 of Sega Force (scan).

Gunstar Heroes (TecToy)
Hoax. Listed on eBay in 2001 along with Thundercats & Primal Rage, but pretty conclusively debunked. In a Retro Gamer interview, the president of TecToy confirmed that it was not released.

Rise of the Robots (Acclaim)
Developed by DataDesign (see here). Game Gear version was released.

Robin Hood: Prince of Thieves (Virgin)
Previewed in Joypad & Hobby Consoles magazines (SMS Power scans: Joypad, Hobby Consoles 1,2) though the accompanying screenshots are taken from the NES version.

Simple FIghter (Sega)
According to issue 4 of 'S' Magazine (March 1990), this was announced, but not shown at the Winter 1990 CES show. Probably a working title for a known game.

Simpsons, The: Bartman vs. Radioactive Man (Acclaim)
Previewed in issue 6 of Sega Master Force (12/93), though the accompanying screenshots are taken from the NES version. GameGear version was released (as "The Simpsons: Bartman meets Radioactiveman")

Sonic Chaos 2 (Sega)
Listed as "In the pipeline" in Sega Power (Jul '94). Released on Game Gear only as 'Sonic Triple Trouble'

Sonic Edusoft (US Gold)
An "unofficially licensed"(?) game supposedly developed by Tiertex in 1991 (Wikipedia entry, according to a mysterious individual claiming to be the programmer. Further intrigue here.

Spider-man/X-Men: Arcade's Revenge (Flying Edge)
Fake-looking box shown in a magazine ad. Released on Game Gear.

Sports Pad Soccer (Sega)
Released in Japan, but also advertised as Coming Soon in the US Spring/Summer 1987 Game Catalog Presumably canned when they decided to add Sports-Pad compatibility to the US Great Soccer instead.

Super Basketball (Sega)
A looping 2-screen demo was shown at a 1989 US trade show (CES?) and later found it's way into Bock's hands.

Bugs Bunny (Sega)
Listed as "coming soon" in "Master System Les Jeux 1992-1993". Game Gear version was released.

This might be a different game since the only other Bugs Bunny game (Bugs Bunny in Double Trouble) was made 3 years later (came out 1996). Possible might be the Crazy Castle game? Not listed in said magazine (just checked at museum).

Prototype name for Deep Duck Trouble (like Donald Duck for Lucky Dime Caper)? Either that or the impossible (port of the NES game? - now that would be a good idea except that it was made by Capcom). Besides if it was a French magazine it should have been La bande a Picsou (as they are more familar with the name).

Oddly I had ideas of porting these games strangely, as for Bartman vs. Radioactive Man might have not made or just concepted as there are NES screenshots.

Thanks for the comments - I've tweaked the list accordingly.
I hadn't checked the dates for the GG Bugs Bunny but you're right - it's unlikely to be the same title. The SMS Bugs Bunny isn't in the main part of the Les Jeux '92/'93 booklet, but is just included on the "A Paraître" list on the inside-back cover.

Also added comments to Arcus Odyssey, Light Corridor & Light Force - they all appeared in the SMS cheats listing in Sega XS magazine, which may be the original source of those rumours.

Bartman vs. Radioactive Man might have not made or just concepted as there are NES screenshots.

Yeah, but in the same issue they also have it down as due for release on the SMS (& GG), the following month from that issue of the mag in their release list for all games due out. So it's not as if they had it as in early development, or due in 6 months - according to them it was due in a few weeks from then. So it would have been WAY past the concept stage. At that timescale you'd think the game was virtually complete! (Not forgetting we know how reliable(!) SMS release lists were at the time!!)

Here's a scan of Robin Hood which might be new (I saw the same screenshots in the other scans, but still). Sega Power #36 (November 1992). The pack contains a hi-res PNG which might come in use for something.

Here's a scan of Robin Hood which might be new (I saw the same screenshots in the other scans, but still). Sega Power #36 (November 1992). The pack contains a hi-res PNG which might come in use for something.

Here are references for two apparently unreleased games for the Master System: Tecmo Cup Football Game and Battletoads. These can be found in a Spanish promo VHS by SEGA from early 1993 (and now uploaded to YouTube).

A couple of pics of Tecmo Cup Football Game:

On screen it reads: "Available" "Mega Drive" "Master Sistem II" (notice the name misspelling, and the console is also referred with the numeral of the second model)

The gameplay footage is from the also unreleased (but eventually leaked) Mega Drive version, but this is the case of every multi-platform game appearing in this VHS.

This can be checked in this YouTube video (first part of the aforementioned VHS), at 2:30.

Notice that this isn't "Battletoads in Battlemaniacs", intended for release in Europe by Virgin in 1994, and eventually released in Brazil by Tec Toy. This is actually the original "Battletoads" NES game, ported and released by SEGA in 1993 for the Mega Drive and Game Gear (this version isn't referenced in the video, though). The gameplay footage, again, is from the Mega Drive version.

Can be checked in this other YouTube video (second part of the same VHS), at 0:42.

Sonic Chaos 2 (Sega)
Listed as "In the pipeline" in Sega Power (Jul '94). Released on Game Gear only as 'Sonic Triple Trouble'

I swear I saw this in my local game hire store (Game Mania, Elizabeth, South Australia) back in the day. The Game Gear was well & truly dead in my area, so it couldn't possibly have that which I saw...

Here's some more stuff for Robin Hood: Prince of Thieves for SMS/GG (that never happened, again it's all NES screenshots). The text mentions that the conversion was (planned to be?) done by Steve Lamb, whose name is on the title screen of Marble Madness; he may be findable (last info I can find placed him at Climax in the UK). Dates are relative to 1993.